Plasma immersion ion implantation is performed by generating a plasma containing ions of species to be implanted in a semiconductor wafer or workpiece. The plasma may be generated using a plasma source such as a toroidal plasma source at the reactor chamber ceiling. Any other plasma source (e.g., RF inductively coupled plasma source) could be used for generating high density plasma above the wafer surface. Ion energy sufficient to achieve a desired ion implantation depth profile below the wafer surface is provided by coupling a high RF bias voltage (e.g., 0.5 kV to 20 kV of capacitively coupled RF plasma) to the semiconductor wafer through an insulated cathode electrode within the wafer support pedestal. If the wafer support pedestal is incorporated within an electrostatic chuck, then the insulated cathode electrode may be a thin metal (e.g., molybdenum) mesh separated from the wafer support surface by a thin (e.g., 1 mm thick) insulation layer. The insulation layer overlying the metal mesh electrode is part of a complete insulation layer within which the metal mesh electrode is encapsulated. The metal mesh electrode is flat in order to provide a uniformly perpendicular electric field across the surface of the wafer. This feature enables the ion trajectory near the wafer surface to be uniformly perpendicular across the entire wafer surface. Such uniformity is needed to achieve uniform distribution of ion implant dose across the wafer surface, leading to uniform distribution of sheet resistance across the wafer surface.
Such a plasma immersion ion implantation process is hampered by non-uniform distribution across the wafer surface of retained implant dose and polysilicon etch rate. The non-uniformity of retained implant dose is reflected in the non-uniform distribution across the wafer surface of measured sheet resistance, the non-uniformity being on the order of about 2-4%. With the recent progress in semiconductor circuit miniaturization, with feature sizes below 45 nanometers, it is becoming imperative that this non-uniformity be reduced to less than 1%. Currently, there has seemed to be no solution to this problem.